Air cells are cells using oxygen in air as active materials and are economical power sources usable for a long period of time without maintenance. In general, there are known button cells each having a structure in which a negative electrode metal case is engaged via a gasket with a positive electrode metal case having air holes. In the internal space of the engaged case body, a negative electrode, a separator, an air electrode (a positive electrode), a water-repellent film and an electrolysis solution are placed. In such a button cell, the internal space of the engaged case body is divided by the separator impregnated with the electrolysis solution. One of the divided spaces serves as the negative electrode filled with zinc, and the other serves as the air electrode (the positive electrode) provided with a catalyst. In addition, the water-repellent film of a polytetrafluoroethylene (PTFE) porous film is placed on the opposite side of the separator on the air electrode side.
Japanese Patent 3034110 discloses that a PTFE porous film composing a water-repellent film is subjected to particular treatment in order to improve water removal performance of the water-repellent film and thereby further extend the life of a button cell.